1. Field of the Invention
The present invention relates to an electromagnetic switch device used mainly for a coaxial electromagnetic switch device used mainly for a coaxial type starter device which starts an engine.
2. Discussion of Background
A coaxial type starter device for starting an engine is disclosed in Japanese Kokai No. 140864/1988, and will be described with reference to FIGS. 6 and 7.
An electromagnetic switch device 2 is attached to the rear end of a dc motor 1 in a coaxial starter device. The electromagnetic switch device 2 slidably moves a rotary output shaft and supplies a current from a battery to the dc motor 1 when a key switch (not shown) mounted on an automobile is closed. The electromagnetic switch device 2 comprises an exciting coil 7 and a plunger 9 as major elements, wherein the exciting coil 7 is formed by winding a wire around a bobbin 6 made of a resinous material held by front and rear cores 4, 5 which form a magnetic path in association with a casing 3. The plunger 9 has a cylindrical body 9a which is slidably disposed in a sleeve 8 fitted to the inner circumference of the bobbin 6.
In the plunger 9, an intermediate plate portion 9b is integrally formed inside the cylindrical body 9a. An end of a plunger rod 10 is connected to the central portion of the intermediate plate portion 9b. The other end of the rod 10 extends in a cylindrical armature rotary shaft from the rear end side of the dc motor 1. The plunger rod 10 transmits the movement of the plunger 9 to the rotary output shaft through an intermediate rod (not shown) disposed in the armature rotary shaft. A sleeve 11 is fitted to the plunger rod 10 at a position in the vicinity of the intermediate plate portion 9b of the plunger 9. A movable contact 13 is held in a slidable manner on the sleeve 11 through an insulating material 12.
In the electromagnetic switch device 2, the rear core 5 has a cylindrical portion 5a which is formed so as to cover the outer circumferential surface of the cylindrical body 9a of the plunger 9 and to be close to the outer circumferential surface of the cylindrical body 9a over its entire length when the plunger 9 is at a stationary position as shown in FIG. 6. The front end portion (an end portion facing the dc motor) of the cylindrical body 5a is formed in such a manner that it overlaps the rear end portion of the cylindrical body 9a in the radial direction when the plunger 9 is entirely shifted in the forward direction (toward the dc motor) as shown in FIG. 7.
The plunger 9 is attracted toward the front core 4 by a magnetic attraction force which is produced by a magnetic flux in a magnetic circuit which passes through the casing 3, the rear core 5, the plunger 9 and the front core 4, the magnetic circuit being produced by current conduction to the exciting coil 7. The movement of the plunger 9 in the forward direction projects the rotary output shaft from the frame of the coaxial type starter device by means of the plunger rod 10 and at the same time, the movable contact 13 held on the plunger rod 10 is brought into engagement with a fixed contact 14 placed at a predetermined position, whereby power is supplied to the dc motor 2.
The magnetic attraction force imparted to the plunger 9 (strictly speaking, to the cylindrical body 9a) in the conventional electromagnetic switch device 1 is shown as a curved line A in a graph as shown in FIG. 5 which shows a relation of the attracting force to a plunger gap (which is a distance indicated by g in FIG. 6). Generally, the magnetic attraction force imparted to the plunger 9 will become larger as the length of the gap g is reduced, i.e., the plunger 9 approaches the entirely shifted state. However, as is clear from the curved line A, a rate of increase of the attracting force decreases. A spring 15 is disposed in the electromagnetic switch device 1 in order to return the plunger 9 to the original stationary position. When the length of the gap g is reduced, the spring 15 is compressed, whereby the plunger 9, receives a great returning force. Accordingly, the magnetic attraction force has to be greater than the spring force in order to return the plunger 9 as the length of the gap g is reduced. In the conventional electromagnetic switch 1, however, the magnetic attraction force imparted to the plunger 9 was insufficient because the rate of increase of the attracting force reduces as the gap g is smaller. This caused a problem in that a predetermined attracting force could not be obtained when a voltage of a power source decreased.